The present invention relates generally to acquisition and calculation of dimensional variables of a workpiece, and specifically to high speed acquisition thereof with vibration and skew error reduction.
In industry vast numbers of parts must be manufactured to within certain tolerance levels, not only commonly referred to tolerances such as diameter and thickness, but also geometric dimensioning and tolerancing. Such geometric dimensioning and tolerancing is used when part features are critical to function or interchangeability, when functional gaging techniques are desirable, when checking to see that the design of a part and a manufactured part are consistent, and the like. Certain characteristics of geometric dimensioning and tolerancing are used to control the features of the part. These characteristics include flatness, straightness, concentricity, circularity, cylindricity, perpendicularity, angularity, parallelism, and the like. These characteristics define various tolerance zones within which the specific conditions of the object being dimensioned must fall. Each characteristic requires specific measurements to be performed on the object in order to determine whether the specific characteristic is met.
Geometric dimensioning and tolerancing standards have been set forth by the American National Standards Institute (ANSI) in ANSI Y14.5M-1994 and the like, which detail how to measure a wide variety of geometric dimensions of an object.
Geometric dimensioning and tolerancing allows for the provision of reliable parts in quantity and with maximum production tolerances. It further confirms whether designed parts are capable of manufacture to exacting standards. It further assists in the use of computerization design techniques. Given the increasing sophistication of products in today""s market place, the tolerances for parts have become increasingly more strict. Geometric dimensioning and tolerancing allows for more accurate and exacting standards of tolerances in the manufacturing process.
Geometric dimensioning also finds use in examination of an existing part where the blueprints or plans for the part may have been lost or are otherwise unavailable. Exacting standards for geometric dimensioning allow for reproducability of a part on the basis of the part itself, as opposed to the plans.
In order to effectively use geometric dimensioning and tolerancing, apparatus and a process to obtain physical parameter data from an object are required. Because of the potentially large quantity of information required in performing geometric dimensioning and tolerancing, speed in measurement techniques, as well as accuracy, is required. Common dimensional measuring techniques include sequential acquisition and reading of dimensional variables of an object, and multiplexing the sequential readings together. However, even in a static environment in which the object is not moving rotationally or otherwise, vibration due to movement of the object, the platform in which the object is resting, and the like may introduce errors into measurements and propagate errors through measurements. In a sequential reading of dimensional variables of an object, when separate sensor readings are taken, a vibrational error may be introduced into the measurements. The result is an unclear representation of the geometry of the piece. This unclear or fuzzy picture will vary depending upon the extent of the ambient vibration. It is nearly impossible to fully attenuate ambient vibrations. Accordingly, even the most exacting of static environment measuring techniques using sequential reading of dimensional variables create imprecise results.
The problems with using sequential reading for measuring dimensional variables are increased even further when the object to be measured is undergoing rotational or translational motion, or the like. Rotation during measurement introduces a skew error no matter how fast the sequential reading is done. This skew error is introduced through time delay. Even if the sampling rate is very fast, measurement of imperfect shapes experiencing rotational motion will have an error component introduced. This skew error is further complicated by ambient vibration which may be by movement of the part or the platform on which the part is resting. Rotational translational and vibrational errors each cause problems, separately, and in combination.
Sequential sampling instrumentation is inherently incapable of taking measurements at two diametrically opposed points on a workpiece in rotational motion. The introduced error will be proportional to the rotation rate of the workpiece. Even when the rotation rate is known, the sampling error cannot be eliminated, only reduced, by reducing the rotation rate of the workpiece. In addition, rotational rate reduction reduces throughput of the system
In performing measurements of a dimensional nature on a workpiece, extremely important factors are repeatability and reproductability. These factors depend on a variety of other factors, including operator variability, skew error, vibrational error, and translational error. Vibrational and translational errors may change each time an object is to be measured. The ambient conditions of vibration will never be identical between any two measuring sequences. Skew error may be affected by dirt on the mounting platform for the object, as well as wear on the object. Operator variability takes into account differences between the way an object is placed for measurement, and operator error.
The measurement of static dimensional variable accuracy is limited by time dependent motions such as vibration that is present in all environments. When measuring dynamic dimensional variable accuracy, it is also limited by intentional and non-intentional time dependent motion. This motion includes rotational, translational, and vibrational motion. Traditional measurement systems minimize the intentional time dependent motion errors by constraining acceleration and velocity of the object to low values. While this solution is partially effective in minimizing intentional time dependent motion errors, it does not address vibrational error. Further, this solution greatly increases the time required to measure the dimensional variable of an object.
It would be desirable to provide method and apparatus for geometric dimensioning and tolerancing which serves to reduce operator variability, while largely eliminating vibrational, translational, and skew errors.
The present invention overcomes the problems of the prior art by providing a high speed acquisition and calculation method and apparatus which reduces vibrational, translational, and skew errors by measuring all inputs necessary to define the variable in the same relative time frame of reference.
With the method and apparatus of the present invention, multiple probes are attached to a fixture, and physically acquire positional information readings on the workpiece simultaneously, in effect creating a snap-shot image of the workpiece at a certain instant. The probes define a surface location on the workpiece by converting the physical location to a signal that can be processed by associated instrumentation operatively connected to the probes. Because all of the probes simultaneously contact and measure the workpiece, rotational skew error and ambient vibrational and translational errors are greatly reduced. The rotational, translational, and vibrational error elements of the measurement are reduced by the simultaneous measurement of the workpiece. When the multiple probes measure the workpiece simultaneously, any ambient vibration will be affecting all of the part equally, so if the part is displaced from a set position, the probes will nevertheless detect only the physical characteristic dimensional data of the workpiece since the probes measure all information simultaneously. Similarly, when the method and apparatus of the present invention obtain dimensional variable information through the simultaneous use of multiple probes, skew error due to rotation of the workpiece at different measurement times and translational error are greatly reduced.
Multiple support structures are used to hold parts being measured. In a constrained support, the part being measured is restrained mechanically. This type of support allows for easy rotational movement of the workpiece. In a freely moving support, the part is placed in a fixture with a certain degree of extra room. In this situation, the part may be movable side to side within the fixture. This motion is translational. If a sequential probe measurement is used, the probe itself may cause the part to move, affecting the accuracy of the measurement. Further, if the part is subjected to vibrational or other forces, it may shift within the fixture between probe contacts, further increasing the measurement error potential. Examples of support types include v-supports, centers, and surface plates which are also referred to as granite plates.
The method of the present invention, as has been mentioned, involves the simultaneous probing of the workpiece by multiple probes in order to determine all relevant dimensional variables associated with the workpiece at the same time. The probes are arranged so that they contact the workpiece in the appropriate positions to measure the dimensional variables required to be measured. For example, in the case of diameter measurements, two probes are used. They may be separated by 180 degrees, but may also be placed in other configurations allowing proper measurement. The diameter of the workpiece is a function of the position of the two probes. If the first probe determines position P1 and the second probe determines position P2, the workpiece diameter is P1+P2. The center point between the two probe positions is (P1xe2x88x92P2)/2.
The multiple probes are used to get raw data readings from the workpiece simultaneously. Each discrete measurement is a snap-shot of the workpiece at the particular time the probes contact the workpiece. With the raw data gathered from the probe readings of the workpiece, multiple form calculations may be made. The raw data may be used to calculate multiple characteristics of the workpiece. This reuse of points allows the method of the present invention to eliminate to a degree unnecessary and time-consuming the repetition of calculations. The effect of this is that calculations may be done much more closely to real time. For example, the method of the present invention can define a line or plane at a particular instant of time. Several levels of calculations may be made in which calculations made previously in the sequence of events may be used to generate further geometric dimensioning information.
These multiple levels of calculations allow for the method of the present invention to provide more complete data in a faster fashion. Virtual gages may be defined from the raw probe position data generated by each discreet measurement. These virtual gages may be used to further calculate different dimensioning characteristics without the need for recalculating from the raw data. In this way, the present invention uses the raw data to increase the calculation speed for determining dimensional characteristics of the workpiece.
Many forms of apparatus exist which will allow the method of the present invention to be embodied. The various probe positions and locations will be determined by the characteristics which are to be measured. Different characteristics require different probe positioning. The discreet nature of the method of the present invention, namely the simultaneous acquisition of dimensional variables, allows certain portions of the probe position information to be used to determine multiple characteristics of the workpiece. The probes are touched to the workpiece to define the surface location by converting the physical location to a signal that can be processed by instrumentation attached to and communicating with the probes. The number of probes used will vary depending upon the specifics of the measurement and the precision required. Probe types may also be varied depending on variables such as physical space available, required stroke length, contact pressure, contact geometry, environmental conditions, the need for pneumatic advance or retract, and the like. The probes relay physical location data to the instrumentation, which in turn performs gaging equations, calibrates and balances the probes, displays the results, and performs further processing as required. The instrumentation may include such electronic components as filters, demodulators, analog to digital (A/D) and digital to analog (D/A) converters, liners, power supplies and the like.